(1) Technical Field
This invention relates to the polishing of semiconductor wafers of the type from which chips for integrated circuits and the like are made. More specifically in a chemical mechanical polishing (CMP) process in which a wafer is held by a tooling polishing head and is polished by contact with an abrasive material dispensed on a rotating polishing pad.
(2) Description of the Prior Art
The fabrication of integrated circuits on a semiconductor wafer involves a number of steps where patterns are transferred from photolithographic photomasks onto the wafer. The photomasking processing steps opens selected areas to be exposed on the wafer for subsequent processes such as inclusion of impurities, oxidation, or etching.
During the forming of integrated circuit structures, it has become increasingly important to provide structures having multiple metallization layers due to the continuing miniaturization of the circuit elements in the structure. Each of the metal layers is typically separated from another metal layer by an insulation layer, such as an oxide layer. To enhance the quality of an overlying metallization layer, one without discontinuities of other blemishes, it is imperative to provide an underlying surface for the metallization layer that is ideally planar.
Conventionally, during the fabrication of integrated circuit structures, planarizing of the overlying metallization layers is accomplished by CMP. The uniform removal of material from and the planarity of patterned and unpatterned wafers is critical to wafer process yield. Generally, the wafer to be polished is mounted on a tooling head which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer and a retaining lip or ring around the edge of the wafer to keep the wafer centered on the tooling head. The front side of the wafer, the side to be polished, is then contacted with an abrasive material such as a polishing pad or abrasive strip. The polishing pad or strip may have free abrasive fluid sprayed on it, may have abrasive particles affixed to it, or may have abrasive particles sprinkled on it.
The ideal wafer polishing process depends on several factors which includes; relative velocity and applied pressure between the wafer and the polishing pad, polishing pad roughness and pad elasticity, surface chemistry, abrasion effects, and contact area. As a result, the ideal CMP process should have constant cutting velocity over the entire wafer surface, sufficient pad elasticity, and a constant supply of polishing slurry. Additionally, control over the temperature and pH is critical and the direction of the relative pad/wafer velocity should be randomly distributed over the entire wafer surface.
Most current CMP machines fail to produce constant velocity distribution over the entire wafer surface which is necessary for uniform material removal and good flatness. A common type, shown in simplified form in FIG. 1, a wafer 10 is held by a tooling head 21 which rotates about the axis of the wafer 10. A large circular polishing pad 13 is rotated while contacting the rotating wafer being held by the tooling head. The rotating wafer contacts the larger rotating polishing pad in an area away from the center of the polishing pad. Thus, the relative motion between the wafer and the polishing pad has two components; one due to the rotating wafer and another due to the rotating polishing pad.
A major disadvantage with prior systems was related to the fact that a wafer moving in a particular direction would have a lower removal rate at its leading edge because of what is commonly referred to as xe2x80x9cedge exclusionxe2x80x9d caused by a ripple formed in the polishing pad 13 when applied pressure between the wafer and the polishing pad is made, refer to FIG. 2. Prior art systems which provide only certain polishing patterns can not be easily made to control the removal rates at the edges due to this effect. As shown in FIG. 2, the ripple formed on the surface of the polishing pad, during contact with the wafer excludes this leading edge segment from contact with the polishing pad, hence, becomes the major contributing factor for edge exclusion. To avoid this problem, see FIG. 3, a fixed floating ring 17 peripherally oriented about the outer edge of the tooling head 21, and fixedly mounted to the tooling head driver housing 18, was developed to displace the ripple in the polishing pad outwardly from the leading edge of the wafer to the leading edge of the fixed floating ring. The addition of the fixed floating ring reduced the effects of edge exclusion, however, its implementation tended to push away the polishing slurry, thereupon, lengthening the polishing cycle.
The primary challenge is to make the oxide removal rate constant across the top surface of the larger diameter wafers, as well as maintaining a constant oxide removal rate during successive wafer runs. Prior polishing techniques do not provide trouble free process control for producing device patterns. The oxide removal rate is not constant between wafers thereby reducing device yield during the fabrication process.
It is therefore a primary object of the present invention to provide a polishing device that can improve the uniformity and planarity of the surface of a wafer being polished by eliminating edge exclusion caused by a ripple occurring on the surface of the polishing pad, and by permitting a more constant availability of polishing slurry as a function of radial distance of the wafer""s contact position on the polishing pad.
According to one aspect of the present invention, there is provided in a polishing device including a rotatable polishing platen having an upper surface on which a polishing pad is attached, a rotatable wafer polishing head assembly having a lower surface opposed to an upper surface of the polishing pad on the polishing plate, for holding a wafer to be polished on the lower surface and pressurizing means for applying a polishing pressure to the rotatable wafer polishing head assembly, whereby the wafer held by the polishing head assembly is rotatably pressed against the rotating upper surface of the polishing pad under the polishing pressure applied from the pressurizing means to perform polishing of the wafer, the improvement wherein the rotatable wafer polishing head assembly is created with a floatable ditch type ring that is coaxial to, yet non-rotating, relative to the rotatable wafer polishing head assembly. The floatable ditch type ring is pneumatically and mechanically urged onto the rotating upper surface of the polishing pad.
It is an object of the present invention to provide a novel process and apparatus for CMP planarization by displacing the ripple on the polishing pad that causes edge exclusion during CMP planarization.
It is another object of the present invention to provide the method and apparatus to enhance the flow of slurry as a function of radial distance of the wafer relative to the rotating polishing plate.
It is still another object of the present invention to provide the method capable of high yield in fabricating a semiconductor device.
It is an additional object of the present invention to provide the method and apparatus useful in a semiconductor device having large scale integration formed on large diameter wafers.
These and further constructional and operational characteristics of the invention will be more evident from the detailed description given hereafter with reference to the figures of the accompanying drawings which illustrate preferred embodiments and alternatives by way of non-limiting examples.